Jackson dated his political conversion to socialism to 1900, after he read a copy of Robert Blatchford's book Merrie England which had been given to him years earlier by an older colleague at the printworks where he had been apprenticed.

That year he joined the Social Democratic Federation, where he developed his oratorical skills at open-air meetings, overcoming the shyness he had endured as a child.

Whilst a member of the SDF, he attended party classes on Marx's Capital which were taught by Jack Fitzgerald, who Jackson described as "very nearly the best-read man I have ever met".

In the early 1920s, he paid visits to Dublin, where he met Constance Markievicz, Charlotte Despard and Maud Gonne, and Moscow, where he was introduced to Joseph Stalin, Sen Katayama and Clara Zetkin, although a planned meeting with Vladimir Lenin was cancelled due to the latter's illness.

In the 1940s, he returned to his roots, working as a lecturer on Communist theory for the Party's Education Department, travelling across the country for eight or nine months of the year.

Jackson's 1935 pamphlet The Jubilee- and How was a critique of the British monarchy, arguing the Silver Jubilee of George V was inappropriate at a time of widespread unemployment.